--- /dev/null
+========================
+ Differences from POSIX
+========================
+
+CephFS aims to adhere to POSIX semantics wherever possible. For
+example, in contrast to many other common network file systems like
+NFS, CephFS maintains strong cache coherency across clients. The goal
+is for processes communicating via the file system to behave the same
+when they are on different hosts as when they are on the same host.
+
+However, there are a few places where CephFS diverges from strict
+POSIX semantics for various reasons:
+
+- If a client is writing to a file and fails, its writes are not
+ necessarily atomic. That is, the client may call write(2) on a file
+ opened with O_SYNC with an 8 MB buffer and then crash and the write
+ may be only partially applied. (Almost all file systems, even local
+ file systems, have this behavior.)
+- In shared simultaneous writer situations, a write that crosses
+ object boundaries is not necessarily atomic. This means that you
+ could have writer A write "aa|aa" and writer B write "bb|bb"
+ simultaneously (where | is the object boundary), and end up with
+ "aa|bb" rather than the proper "aa|aa" or "bb|bb".
+- POSIX includes the telldir(2) and seekdir(2) system calls that allow
+ you to obtain the current directory offset and seek back to it.
+ Because CephFS may refragment directories at any time, it is
+ difficult to return a stable integer offset for a directory. As
+ such, a seekdir to a non-zero offset may often work but is not
+ guaranteed to do so. A seekdir to offset 0 will always work (and is
+ equivalent to rewinddir(2)).
+- Sparse files propagate incorrectly to the stat(2) st_blocks field.
+ Because CephFS does not explicitly track which parts of a file are
+ allocated/written, the st_blocks field is always populated by the
+ file size divided by the block size. This will cause tools like
+ du(1) to overestimate consumed space. (The recursive size field,
+ maintained by CephFS, also includes file "holes" in its count.)
+- When a file is mapped into memory via mmap(2) on multiple hosts,
+ writes are not coherently propagated to other clients' caches. That
+ is, if a page is cached on host A, and then updated on host B, host
+ A's page is not coherently invalidated. (Shared writable mmap
+ appears to be quite rare--we have yet to here any complaints about this
+ behavior, and implementing cache coherency properly is complex.)
+- CephFS clients present a hidden ``.snap`` directory that is used to
+ access, create, delete, and rename snapshots. Although the virtual
+ directory is excluded from readdir(2), any process that tries to
+ create a file or directory with the same name will get an error
+ code. The name of this hidden directory can be changed at mount
+ time with ``-o snapdirname=.somethingelse`` (Linux) or the config
+ option ``client_snapdir`` (libcephfs, ceph-fuse).
Code Review / stor4nfv.git / blobdiff